nir/validate: Use the type from the tail of call parameter derefs

Otherwise, if what gets passed into the function call is a deref chain
longer than just a variable deref, we would use the type of the entire
variable rather than the type of the thing being dereferenced.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106980


Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(Unique to 18.1)